The starting point for the invention was the continuing demand for an effective and economical method of producing high-purity silica.
High-purity silica can be used for making so-called solar-grade silicon, which is required for the production of photovoltaic cells, and is therefore of enormous economic importance.
An advantageous method of producing high-purity silica using silicate solutions is described in document WO 2010/037702 A1. However, this method has the disadvantage that the silica obtained must undergo an expensive washing process and the washing medium used for this must be strongly acidic. The pH values of the washing acids used for silica washing must be below 2, especially preferably even below 0.5, and even at these low pH values, di- and higher valent metallic impurities are still adsorbed so strongly on the surface of the silica particles that large amounts of strongly acidic washing acid are required for cleaning. For this reason the existing method is not only cost-intensive, it also involves risks with respect to labour safety, especially when replacing the washing acid, especially as this process step also has a concomitant risk of introduction of impurities.
Another drawback of the method according to document WO 2010/037702 A1 is that contaminations of the silica with titanium ions in the conditions of the acidic washing process cannot be removed effectively. To tackle this problem, preferably a peroxide is added, which not only makes it possible to remove the titanium ions by complexation, but owing to the colour of the complex formed it also makes the progress of its removal visible.
However, peroxides in acidic media have the disadvantage that hydroxyl radicals form in the presence of iron(II) ions, which are usually present in the process (Fenton's reagent). These radicals constitute a very strong oxidizing agent and, because of their corrosive effects, require especially high-grade materials for the equipment, making the production process even more expensive.
One problem to be solved by the present invention was therefore to provide a method of producing high-purity silica that is less cost-intensive compared with the prior art. In particular the method should be improved so that the high consumption of acid for the expensive washing process can be reduced. Furthermore, this washing process should—also owing to the risks associated with the handling of strongly acidic media—be simplified. Finally, another problem was to improve the method to the extent that addition of peroxides is no longer required.